a) Field of the Invention
The present invention relates to a zoom lens system for use with photographic cameras and so on.
b) Description of the Prior Art
In the recent years, zoom lens systems are generally used as lens systems for photographic camera since the zoom lens systems have merits to permit changing image sizes relatively freely and allow photographing variations in broad ranges. Though most of the zoom lens systems are currently designed for vari-focal ratios of 2 to 3, it is anticipated that zoom lens systems having higher vari-focal ratios of 4 to 6 will hereafter be demanded. As a vari-focal ratio becomes higher, however, a zoom lens system comprises a larger number of lens elements and has larger dimensions, thereby increasing weight thereof and enhancing manufacturing cost therefor.
In addition to homogenous spherical lens elements, homogenous aspherical lens elements and graded refractive index lens elements (GRIN lenses) are used for composing zoom lens systems. Since these homogenous aspherical lens elements and GRIN lenses have higher flexibility for correcting aberrations than the homogenous spherical lens elements, it is possible by using the homogenous aspherical lens elements and GRIN lenses to reduce numbers of lens elements composing zoom lens systems, thereby making zoom lens systems compacter, lighter in weights and manufacturable at lower costs.
As examples of lens systems using aspherical lens elements which have already been put to practical use, there are known the optical pickup single-element lens components and the lens systems for compact cameras composed of four groups of four lens elements such as that disclosed by Japanese Examined Published Patent Application No. 9607/61.
The graded refractive index lenses are classified into the so-called axial GRIN lens having refractive index gradient in the direction along the optical axis and the so-called radial GRIN lens having refractive index gradient in the direction perpendicular to the optical axis. Out of these GRIN lenses, especially the radial GRIN lens is capable, when used as a single lens element, of correcting Petzval's sum and chromatic aberration which cannot be corrected with a single homogenous lens element as reported by Applied Optics Vol. 7, No. 7 page 1081, Applied Optics Vol. 21, No. 6, page 992 and so on.
As examples of the lens systems using the radial GRIN lenses, there are known the lens systems disclosed by Japanese Unexamined Published Patent Applications No. 220115/58 and No. 218614/60. These lens systems comprise lens elements in numbers far smaller than those of the lens elements used in the lens systems composed only of homogenous spherical lens elements. However, the lens systems disclosed by the above-mentioned patent Applications have not been put to practical use yet since the GRIN lenses used therein have problems from the viewpoint of manufacturing.
In order to put the GRIN lenses to practical use, it is necessary to solve the problems on manufacturing and conceive some ways of efficient employment which can sufficiently make use of the characteristics of the GRIN lenses.